Based on the commit message and using "CL" which is google lingo for Change List on their internal system, I bet this was already available on the internal version and just ported to github version after someone pointed it out.
Much more prosaic (if slightly embarrassing), I'm afraid: The update was non-trivial (this CL is simple, but there are some accompanying ones in x/text which are not) and it didn't hit the top of the priority list for anyone who understands x/text.
Go is pretty much entirely developed in public; there are some Google-internal customizations but none of them are particularly exciting and almost all changes start in the open source repo and are imported from there.
"CL"/"Change List" is the lingo for the Gerrit code review tool, which is how all contributions to Go happen. Creating a GitHub PR simply triggers a bot to create a Gerrit CL, which is where all discussion about the "PR" happens and where the "accept" button gets clicked.
